The invention relates to communications systems generally and, more particularly, the invention relates to a method and concomitant apparatus for adaptively encoding an information stream in response to indicia of an information stream discontinuity.
In several communications systems the data to be transmitted is compressed so that the available bandwidth is used more efficiently. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172 and is incorporated herein by reference. The second, known as. MPEG-2, refers to ISO/IEC standards 13818 and is incorporated herein by reference. A compressed digital video system is described in the Advanced Television Systems Committee (ATSC) digital television standard document A/53, and is incorporated herein by reference.
The above-referenced standards describe data processing and manipulation techniques that are well suited to the compression and delivery of video, audio and other information using fixed or variable length digital communications systems. In particular, the above-referenced standards, and other xe2x80x9cMPEG-likexe2x80x9d standards and techniques, compress, illustratively, video information using intra-frame coding techniques (such as run-length coding, Huffman coding and the like) and inter-frame coding techniques (such as forward and backward predictive coding, motion compensation and the like). Specifically, in the case of video processing systems, MPEG and MPEG-like video processing systems are characterized by prediction-based compression encoding of video frames with or without intra- and/or inter-frame motion compensation encoding.
In a typical MPEG encoder, a received video stream comprising a plurality of video frames is encoded according to a predefined group of pictures (GOP) structure. That is, the received video stream is encoded to produce a GOP comprising, e.g., an intra-coded frame (I-frame), followed by one or more forward predicted coded frames (P-frames) and bi-directional (i.e., forward and backward) predicted frames (B-frames). In the case of a scene change in the received video stream, the first frame of the new scene may be significantly different than the previous anchor frame. Thus, the encoder may need to intra-code a very large percentage of the macroblocks in the first frame. In this situation, encoders typically encode the frame as an anchor frame, from which subsequent frames within the predefined GOP structure will be predicted.
Unfortunately, if the new anchor frame was targeted to be coded as a P-frame, its intra-coding impacts the rate control (RC) predictions utilized by the encoder. Depending on how RC is done, this can affect the coded quality of the P-frame itself and the quality over the few frames that are encoded after the P-frame. In particular, the effect is felt the most when the previous anchor frame was an I-frame.
In addition, unless the first anchor frame after a scene change is declared an I-frame, rather than a P-frame with most or all of its macroblocks intra-coded, random access (i.e., independent decodability) is not gained near the start of the scene change. Thus, to retain random access, some encoders simply code the first frame after a scene change as an I-frame, whether it was scheduled to be a P-frame or a B-frame. Such encoder behavior also impact the RC behavior of the encoder.
Therefore, it is seen to be desirable to address the above-described problems by providing a method and concomitant apparatus for adapting the behavior of an MPEG-like encoder to scene changes within a received video stream such that encoding quality and random access to the encoded stream is retained near scene change points. More generally, it is seen to be desirable to provide a method and concomitant apparatus for adapting the behavior of an MPEG-like encoder to information discontinuities within a received information stream of any type, such that encoding quality and random access to the encoded stream is retained near information discontinuity points.
The invention comprises a method and concomitant apparatus for adapting the behavior of an MPEG-like encoder to information discontinuities within a received information stream, such that encoding quality and random access to a resulting encoded stream is retained near information discontinuity point without adversely impacting buffer utilization parameters. Specifically, an anchor frame comprising an I-frame preceding an information discontinuity is encoded as a P-frame, while an anchor frame following the information discontinuity is encoded as an I-frame.
Specifically, in a system compression coding a sequence of unencoded information frames to produce a sequence of encoded information frames substantially in accordance with a group of frames (GOF) information structure, each GOF comprising at least one sub-GOF, each sub-GOF comprising at least anchor frame, each anchor frame comprising one of an intra-coded frame (I-frame) and a forward predicted frame (P-frame), a method according to the invention comprises the step of adapting, in response to an inter-frame information discontinuity within the sequence of unencoded information frames, the GOF information structure such that a first anchor frame following the information discontinuity comprises an I-frame, and a first anchor frame preceding the information discontinuity comprises a P-frame.